Optical storage media are media in which data are stored in an optically readable manner, for example by means of a pickup comprising a laser for illuminating the optical storage medium and a photo-detector for detecting the reflected light of the laser beam when reading the data. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media as well as rewritable formats like CD-RW, DVD-RW, DVD+RW and DVD-RAM for example. Digital data are stored in these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store 50 GB on a dual layer disc. Available formats are at present read-only BD-ROM, re-writable BD-RE and write once BD-R discs. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T, maximum 9T is used, where T is the channel bit length, which corresponds with a minimum mark length of 138-160 nm. Further information about the Blu-Ray disc system is available for example from the Blu-Ray group via Internet: www.blu-raydisc.com.
New optical storage media with a super resolution near-field structure (Super-RENS) offer the possibility to increase the data density of the optical storage medium by a factor of three or four in one dimension in comparison with the Blu-Ray disc. This is possible by using a so-called Super-RENS structure or layer, which is placed above an information layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The super resolution layer is also called a mask layer because it is arranged above the data layer and only the high intensity center part of a laser beam can penetrate the mask layer.
The Super-RENS effect allows to record and read data stored in marks of an optical disc, which have a size below the resolution limit of a laser beam used for reading or writing the data on the disc. As known, the diffraction limit of the resolution of the laser beam is about lambda/(2×NA), where lambda is the wavelength and NA the numerical aperture of the objective lens of the optical pickup.
A Super-RENS optical disc comprising a super resolution near-field structure formed of a metal oxide or a polymer compound and a phase change layer formed of a GeSbTe or a AgInSbTe based structure for recording of data and reproducing of data is known from WO 2005/081242 and US 2004/0257968. Further examples of super-resolution optical media are described in WO 2004/032123 and by Tominaga et al., Appl. Phys. Lett. Vol. 73, No. 15, 12 Oct. 1998. The super RENS effect allows to increase the resolution of the optical pickup for reading of the marks on an optical disc, but does not allow to reduce the track pitch.
In Marx and Psaltis, “Optical diffraction of focused spots and sub-wavelength structures” Journal of Opt. Soc. Am. A/Vol. 14, No. 6/June 1997, the diffraction behavior of pits of a proposed new digital video disc format is discussed. According to a numerical analysis, the phase depth of the pits of an optical disc depends strongly on the polarization of the incident optical beam. For pit widths between λ/2 and λ, where λ is the wavelength of the incident light, the phase depth for TM polarized light remains essential constant, but shows a strong dependence on the pit widths for TE polarized light. The results are explained in correspondence with a wave guide model, where for d<λ/2 only evanescent TE wave can exist and only a TM zero mode can propagate. For the measurement of each polarization TE, TM independently, an optical set up is proposed including a polarizing beam splitter in the return parts and a standard detector array for each polarization component.